The Art of Pottery
In medieval times sand was mixed with clay to make cooking pots strong enough to be placed over an open fire. Today, for the same reason, casseroles used for baking are made from clay mixed with grog which is a ground-up fired pottery. The openness of grog clay allows water to evaporate more evenly as it dries and prevents cracking and warping during the firing. Grog clay eases the problem of heat expansion which can cause large thick pieces of pottery or sculpture to blow up in the kiln. Around the middle of the thirteenth century German potters started to produce stoneware. This pottery was made form finer clays and fired at a higher temperature than earthenware. Stoneware was tan or gray in color, strong and naturally non-porous.
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